Spelling correction in xapian bridge

Xapian bridge now performs spelling correction
on a users query and returns the correctedQuery
(which could be an empty string) as part of the
JSON response to client.

[endlessm/eos-sdk#1982]
Recommend replacing `correctedResults` with a name that indicates the connection to spelling (e.g., `spellCorrectedResults`). The rationale here is that there are a lot of different ways of correcting queries (e.g., synonym substitution, stop word removal, stemming, case normalization, structured-data normalization).